300TE transmission query

[adrianruk]

A few weeks ago I bought a '90 300TE with 192000m. I failed to check the transmission oil before purchase - a bad mistake. When I did check I found it was full of emulsified oil. I drained it, changed the filter & refilled it with Dexron II. I ran the car for a couple of days then drained & refilled the oil. Until then the trans had been working fine. Now its OK when the car is cool but when hot the changes up and down are very sharp and the gears engage with a hard clunk. Also, when I put it in reverse there is a lot of resistance, almost as though the brakes are on and I have to give it a lot of throttle just to get it to move. If I leave it for 10 mins or so to cool down every thing is fine again - until it warms up.
I have today drained the oil and found it clean but it had changed colour from pink to a dark reddish brown. I had the rear casing off to change the speedo drive gear which broke a week or so ago (don't know if this was related or just coincidence!). Everything visible appeared fine.
The car had new coolant pipes fitted 9 months ago and has recently had a new radiator fitted ( don't know exactly when).
Any help would be gratefully received.
It has a 722/3 transmission.
I'm trying to avoid having to fit a rebuilt unit
I am toying with fitting a secondary oil cooler since the box seems fine at lower temperatures. Or perhaps I should add some water to the oil since it was OK before I changed it!!!
My '87 300TE is still going strong at 265000m & has caused me hardly any problems in the 4 yrs and 140000 since I bought it. Perhaps I should keep it & sell the new one.
Adrian
PS I'm also having uneven idle/stalling problems which I'm working my way through the archives for the solution.